Semiconductor memory device

ABSTRACT

A semiconductor memory device includes a charge pumping circuit, a level sensor, an oscillator, and a pumping control signal generator. The charge pumping circuit performs a negative-pumping operation to an external power in order to generate an internal voltage having a level lower than the external power. The level sensor senses a level of the internal voltage corresponding to a level of an adjusted reference voltage during a refresh mode. The oscillator generates a period signal in response to a sensing signal of the level sensor. The pumping control signal generator controls the operation of the charge pumping circuit in response to the period signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.11/819,804 filed on Jun. 29, 2007 now U.S. Pat. No. 7,609,566 whichclaims priority of Korean patent application number 10-2007-0000409filed on Jan. 03, 2007. The disclosure of each of the foregoingapplications is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to semiconductor design technologies; and,more particularly, to a semiconductor memory device which is providedwith an internal power generator capable of increasing a cell retentiontime during a self refresh mode and a refresh unit capable of minimizingpower consumption by adjusting a refresh period depending on a level ofan internal power.

Generally, basic operations of a semiconductor memory device include awrite operation for storing data provided from outside and a readoperation for outputting desired data stored therein upon request fromthe outside. In order to perform these write and read operations, thesemiconductor memory device requires the capability of storing data fromthe outside.

In particular, since DRAM (Dynamic Random Access Memory) is anonvolatile memory, data stored therein is lost after a fixed amount oftime elapses. Therefore, a refresh operation is needed to fully restoredata stored in memory cells before the fixed amount of time in order toprevent the loss of data. This refresh operation has priority over anyother operations in DRAM.

Such a refresh operation is done at a fixed period that is closelyconcerned with a retention time of cell data. More details on this willbe given below with reference to a circuit and a cross-sectional view ofunit memory cell.

FIG. 1A is a conceptual circuit diagram of unit memory cell of aconventional DRAM. For reference, the unit memory cell denotes a spacewhere 1 bit data is stored.

Referring to FIG. 1A, the unit memory cell is provided with a capacitorC1 for storing data and an NMOS transistor NM1 for accessing thecapacitor.

To be more specific, a word line WL is connected to the gate end of theNMOS transistor NM1. And, a bit line BL is connected to one side activeregion (here, a drain end) of the NMOS transistor NM1 and the cellcapacitor to other side active region (here, a source end) thereof.

FIG. 1B is a cross-sectional view of the unit memory cell of FIG. 1A,and particularly shows a leakage current generated in the unit memorycell.

As shown in FIG. 1B, although the transistor is turned off, a leakagecurrent occurs in the cell capacitor (here, it is assumed that logichigh data is stored in the cell capacitor). Two major factors of theleakage current are an off current I_(OFF) and a junction currentI_(JUN).

Meanwhile, an internal power VBB with a negative electric potentiallower than a ground voltage is generally connected to a bulk of the NMOStransistor in the unit memory cell shown in FIG. 1A. By keeping the bulkbias low like this, the leakage current as shown in FIG. 1B is reducedby setting a threshold voltage of the transistor higher than that of ageneral NMOS. But, as the level of the internal power VBB lowers, thethreshold voltage becomes higher to decrease off-current, while ratherincreasing the leakage current by elevation of junction-current. Thatis, the off-current and the junction-current constituting the leakagecurrent have a trade-off relationship depending on the level of theinternal power VBB. Therefore, in order to extend a retention time ofcell data, it is important to find an optimal level of the internalpower VBB making both the off-current and the junction-current smaller.

The following is a description for a conventional internal powergenerator for generating an internal power VBB to be applied to a bulkend of cell and a refresh unit for refreshing cell data.

FIG. 2 is a block diagram showing a configuration of a conventionalinternal power generator.

Referring to FIG. 2, the conventional internal power generator includesa charge pumping circuit 40 for negative-pumping an external power VDDto generate an internal power VBB with a level lower than the externalpower VDD, a reference voltage generator 50 for producing a referencevoltage VINT_BB, a level sensor 10 for sensing a level of the internalpower VBB on the basis of the reference voltage VINT_BB, an oscillator20 for generating a period signal OSC in response to a sensing signalBBE of the level sensor 10, and a pumping control signal generator 30for controlling the operation of the charge pumping circuit 40 inresponse to the period signal OSC.

The reference voltage generator 50 is provided with a voltage generator52 for generating a target voltage of the internal power VBB and a levelshifter 54 for level-shifting an output voltage VREF of the voltagegenerator 52 to generate the reference voltage VINT_BB having a stablelevel regardless of the external power VDD (or power supply voltage).

FIG. 3 illustrates an internal circuit diagram of the level sensor 10 ofFIG. 2.

Referring to FIG. 3, the level sensor 10 is composed of a voltagedivider 12 for voltage-dividing a level difference between the groundvoltage VSS and the reference voltage VINT_BB by a level differencebetween the ground voltage VSS and the internal power VBB, an inverter14 for taking the reference voltage VINT_BB and the ground voltage VSSas driving powers and inverting and outputting an output voltage of thevoltage divider 12, a differential amplifier 16 taking an output voltageof the inverter 14 and an inverted voltage of the output of the inverter14 as differential inputs, and an inverter I1 for inverting an outputvoltage of the differential amplifier 16 to provide an inverted voltageas the sensing signal BBE.

Hereinafter, the operation of the internal power generator shown inFIGS. 2 and 3 will be briefly described.

First, the level sensor 10 senses a level of a feedbacked internal powerVBB on the basis of the reference voltage VINT_BB. At this time, whenthe level of the internal power VBB is higher than the reference voltageVINT_BB and the output voltage of the voltage divider 12 exceeds a logicthreshold voltage of the inverter 14, the sensing signal BBE isactivated to a logic high level.

Then, the oscillator 20 is active by the sensing signal BBE to createthe period signal OSC. In response to the period signal OSC, the pumpingcontrol signal generator 30 drives the charge pumping circuit 40, whichcauses the level of the internal power VBB to drop.

When the level of the internal power VBB drops, the output voltage ofthe voltage divider 12 becomes lower than the logic threshold voltage ofthe inverter 14 which makes the sensing signal BBE inactivated to alogic low level.

Thus, the operations of the oscillator 20, the pumping control signalgenerator 30 and the charge pumping circuit 40 are closed.

As described above, the internal power generator in the conventionalsemiconductor memory device is driven to maintain the internal power VBBin a level corresponding to the target level of the reference voltage.Here, as mentioned above, the level of the reference voltage is set suchthat the operation such as storage of data in write operation orrestoration of data in read operation can be made within a designatedtime, while securing the retention time of data by reduction in leakagecurrent. For reference, in order to secure the retention time of data,it is preferred that the level of the internal power VBB is as low aspossible. But, if the level of the internal power VBB becomes lower, thethreshold voltage becomes higher, which prolongs the operation time ofdata storage or restoration.

However, the conventional internal power generator is driven withoutconsidering IDD6 circumstance capable of securing a greater margin thanan active mode as a driving time for restoration of data. That is, itwas impossible to control the retention time of cell data to be extendedfor the refresh interval.

For reference, the IDD6 circumstance is a mode that is entered when theclock enable signal CLK is transited to a logic low level and againstores all cells by performing 8K number of times of refreshes for 64ms.

FIG. 4 is a block diagram showing a configuration of a refresh unitincluded in the conventional semiconductor memory device.

Referring to FIG. 4, the conventional refresh unit includes a modeinput/output controller 60 for accepting a clock enable signal CKE andan auto refresh command AREF_CMD and generating an internal auto refreshsignal AREFP, a self refresh entry signal SREF_EN and a self refreshescape signal SREF_EXP, a refresh interval signal generator 70 forgenerating a self refresh interval signal SREF notifying a self refreshinterval by using the internal auto refresh signal AREFP, the selfrefresh entry signal SREF_EN and the self refresh escape signalSREF_EXP, a refresh period signal generator 80 for periodicallyoutputting a period-pulse signal PL_FLG during activation of the selfrefresh interval signal SREF, an internal refresh signal generator 90for activating an internal refresh signal REFP in response to theinternal auto refresh signal AREFP and the period-pulse signal PL_FLG,and an internal address counter 95 for increasing a row address by onebit unit in response to the internal refresh signal REFP to output aninternal address RCNTI[0:N].

For reference, the clock enable signal CKE is a signal representingwhether a clock synchronizing the operation of the semiconductor memorydevice is valid or not. Thus, if only the clock enable signal CKE isinactivated, the semiconductor memory device enters a power-down modefor minimizing its own power consumption.

FIG. 5 shows an internal circuit diagram of the refresh period signalgenerator 80 of FIG. 4.

Referring to FIG. 5, the refresh period signal generator 80 includes anoscillator 82 which has an inverter chain and is active upon activationof the self refresh interval signal SREF to generate a signal OSC_OUT atregular intervals, and a pulse generator 84 for producing theperiod-pulse signal PL_FLG of pulse type from the output signal OSC_OUTof the oscillator 82.

In brief operation, first of all, when the self refresh interval signalSREF is activated to a logic high level, the oscillator 82 generates thesignal OSC_OUT at a regular interval. Here, the regular intervals aredetermined based on the voltage levels of signals applied to the gateends of NMOS transistors and PMOS transistors constituting the inverterchain. Next, the pulse generator 84 senses a rising edge of the outputsignal OSC_OUT of the oscillator 82, and generates the period-pulsesignal PL_FLG of pulse type.

FIG. 6 presents an operational waveform diagram of the refresh unitaccording to the prior art shown in FIGS. 4 and 5.

As shown in FIG. 6, the clock enable signal CKE is first transited to alogic low level and at the same time the auto refresh command AREF_CMDis activated. Then, the mode input/output controller 60 activates theself refresh entry signal SREF_EN in response to the logic leveltransition of the clock enable signal CKE and activates the internalauto refresh signal AREFP in response to the auto refresh command AR.

Next, the internal refresh signal generator 90 generates the internalrefresh signal REFP in response to the internal auto refresh signalAREFP. In succession, the internal address generator 95 increases therow address by one bit unit whenever the internal refresh signal REFP isactivated, to output the internal address RCNTI[0:N].

Further, the refresh interval signal generator 70 activates the selfrefresh interval signal SREF in response to activation of the internalauto refresh signal AREFP and the self refresh entry signal SREF_EN,wherein this activation is maintained until the self refresh escapesignal SREF_EXP is applied.

Subsequently, the refresh period signal generator 80 periodicallyactivates the period-pulse signal PL_FLG during the activation of theself refresh interval signal SREF. And then, the internal refresh signalgenerator 90 activates a new internal refresh signal REFP of pulse typewhenever the period-pulse signal PL_FLG is applied. Lastly, the internaladdress generator 95 increases the row address by one bit unit wheneverthe internal refresh signal REFP is activated, to output the internaladdress RCNTI[0:N].

For reference, the internal refresh signal REFP is applied to each bank,which makes a word line corresponding to the internal address RCNTI[0:N]active to perform self refresh.

Meanwhile, the refresh period by the refresh unit in the conventionalsemiconductor memory device is determined by the period of theperiod-pulse signal PL_FLG. The period-pulse signal PL_FLG is generatedat a regular period, regardless of the level of the internal power VBB.Therefore, although the level of the internal power VBB is optimized sothat the retention time is reduced, it is unlikely to reflect the above.This reduces the number of times of refresh and thus cannot decreasepower consumption.

Therefore, the conventional semiconductor memory device does not adjustthe level of the internal power under the self refresh mode, therebymaking it impossible to adjust the retention time of cell data. Also,the refresh unit cannot be driven appropriately according to theretention time.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to provide asemiconductor memory device for reducing a current consumption byadjusting a refresh period according to a bulk voltage level.

In accordance with an aspect of the present invention, there is provideda semiconductor memory device including a charge pumping circuit, alevel sensor, an oscillator, and a pumping control signal generator. Thecharge pumping circuit performs a negative-pumping operation to anexternal power in order to generate an internal voltage having a levellower than the external power. The level sensor senses a level of theinternal voltage corresponding to a level of an adjusted referencevoltage during a refresh mode. The oscillator generates a period signalin response to a sensing signal of the level sensor. The pumping controlsignal generator controls the operation of the charge pumping circuit inresponse to the period signal.

In accordance with another aspect of the present invention, there isprovide driving method of a semiconductor memory device includingperforming a negative-pumping operation to an external power in order togenerate a bulk voltage of memory cell; sensing a level of the bulkvoltage in response to a reference voltage; and controlling thenegative-pumping operation so that the bulk voltage has a levelcorresponding to the reference voltage. The reference voltage havedifferent voltage levels for a self refresh mode and a normal mode.

In accordance with a further another aspect of the present invention,there is provided a semiconductor memory device including a refreshexit/entry controller and a control signal generator. The refreshexit/entry controller receives a clock enable signal and an auto refreshcommand and generates a self refresh interval signal notifying that thecurrent operation is in a self refresh mode. The control signalgenerator periodically generates an internal refresh signal and aninternal address for refresh driving during activation of the selfrefresh interval signal. An activation period of the internal refreshsignal is adjusted according to a level of a bulk voltage applied to amemory cell.

In accordance with still another aspect of the present invention, thereis provided a driving method of a semiconductor memory device, includingoperating the semiconductor memory device in a self refresh mode byactivating a self refresh interval signal in response to a clock enablesignal and an auto refresh command; and generating an internal refreshsignal and an internal address for controlling refresh driving atpredetermined intervals during an activation of the self refreshinterval signal. The self refresh interval signal notifies that acurrent operation is performed in the self refresh mode. Thepredetermined intervals are adjusted according to a level of a bulkvoltage applied to a memory cell.

In accordance with still another aspect of the present invention, thereis provided a semiconductor memory device, including an internal powergenerator, a refresh unit, a level controller, a period adjustor. Theinternal power generator performs a negative-pumping operation to anexternal power in order to generate a bulk voltage to be applied to abulk end of cell. The refresh unit generates an internal refresh signalfor refresh driving at regular intervals during a self refresh mode. Thelevel controller controls a level of the bulk voltage in the selfrefresh mode. The period adjustor adjusts the regular intervals when thelevel controller is driven.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a conceptual circuit diagram of unit memory cell of aconventional DRAM.

FIG. 1B is a cross-sectional view of the unit memory cell of FIG. 1A,and particularly shows a leakage current generated in the unit memorycell.

FIG. 2 is a block diagram showing a configuration of a conventionalinternal power generator.

FIG. 3 is an internal circuit diagram of the level sensor of FIG. 2.

FIG. 4 is a block diagram showing a configuration of a refresh unit in aconventional semiconductor memory device.

FIG. 5 is an internal circuit diagram of the refresh period signalgenerator of FIG. 4.

FIG. 6 is an operational waveform diagram of the refresh unit accordingto the prior art shown in FIGS. 4 and 5.

FIG. 7 is a block diagram illustrating a configuration of asemiconductor memory device in accordance with a preferred embodiment ofthe present invention.

FIG. 8 is an internal circuit diagram of the internal voltage generatorof FIG. 7.

FIG. 9 is an internal circuit diagram of the level sensor of FIG. 8.

FIG. 10 is an internal circuit diagram of the refresh unit of FIG. 7.

FIG. 11 is an internal circuit diagram of the refresh period signalgenerator of FIG. 10.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings so thatthe invention can be easily carried out by those skilled in the art.

FIG. 7 is a block diagram illustrating a configuration of asemiconductor memory device in accordance with a preferred embodiment ofthe present invention.

Referring to FIG. 7, the inventive semiconductor memory device includesan internal power generator 100 for negative-pumping an external powerto generate an internal power VBB to be applied to a bulk end of cell, arefresh unit 500 for generating an internal refresh signal REFP forrefresh driving at regular intervals during a self refresh mode, a leveladjustor 240 for adjusting a reference level of the internal power VBBin the self refresh mode, and a period adjustor 722 for adjusting aninterval of the internal refresh signal when the level adjustor 240 isdriven.

As stated above, the semiconductor memory device of the inventionadditionally includes the level adjustor 240 adjusts the level of theinternal power VBB during the self refresh mode, and adjusts the levelof the internal power VBB so that a minimum leakage current is generatedfrom memory cell during the self refresh mode. Further, thesemiconductor memory device additionally includes the period adjustor722 for adjusting an execution interval of refresh by sensing the leveladjustment of the internal power VBB, to thereby reduce the number oftimes of refresh performed during the self refresh mode. Thus, IDD6,that is, power consumed during the self refresh mode can be reduced.

The following is a detailed description for an internal circuit diagramof each block with reference to the drawings.

FIG. 8 is an internal circuit diagram of the internal voltage generator100 of FIG. 7.

Referring to FIG. 8, the internal power generator 100 is provided with alevel sensor 200 for sensing a level of the internal power VBBcorresponding to a level of an adjusted reference voltage VINT_BB duringthe refresh mode, and a power supplier 300 for negative-pumping anexternal power VDD under the control of the level sensor 200 to generatethe internal power VBB having a level lower than the external power VDD.

More specifically, the power supplier 300 is composed of a chargepumping circuit 360 for negative-pumping the external power VDD togenerate the internal power VBB having a level lower than the externalpower VDD, an oscillator 320 for generating a period signal OSC inresponse to a sensing signal BBE of the level sensor 200, and a pumpingcontrol signal generator 340 for controlling the operation of the chargepumping circuit 360 in response to the period signal OSC.

For reference, the internal power generator 100 is further provided witha reference voltage generator 400 for generating the reference voltageVINT_BB having a stable level regardless of the external power VDD. Thereference voltage generator 400 is composed of a voltage generator 420for producing a target voltage of the internal power VBB and a levelshifter 440 for level-shifting an output voltage of the voltagegenerator 420 to create the reference voltage VINT_BB having a stablelevel regardless of the external power (power supply voltage) VDD.

FIG. 9 is an internal circuit diagram of the level sensor 200 of FIG. 8.

Referring to FIG. 9, the level sensor 200 is provided with a feedbackcircuit 220 for providing a level of the internal power VBB on the basisof the reference voltage VINT_BB as a feedback voltage, a levelcontroller 40 for controlling a level of the feedback voltage during theself refresh mode, and a differential amplifier 260 for taking thefeedback voltage and its inverted feedback voltage as differentialinputs to output the sensing signal BBE.

The level controller 240 is composed of an NAND gate ND1 taking a selfrefresh interval signal SREF notifying that the current operation is inthe self refresh mode and a first level adjustment signal VBB_UPP, aninverter I1 for inverting an output signal of the NAND gate ND1 toprovide an up-control signal CRT_UP, an NAND gate ND2 taking the selfrefresh interval signal SREF and a second level adjustment signalVBB_DN, and an inverter I2 for inverting an output signal of the NANDgate ND2 to output a down-control signal CRT_DN.

The feedback circuit 220 is composed of first to fourth PMOS transistorswhose each gate takes a ground voltage VSS and which are connected inseries between the reference voltage VINT_BB and an output node, fifthto eighth PMOS transistors whose each gate takes the internal voltageVBB and which are connected in series between the output node and theground voltage, a first NMOS transistor NM2 whose gate accepts thedown-control signal CRT_DN and which is connected in parallel with thefirst PMOS transistor, and a second NMOS transistor NM3 whose gatereceives the up-control signal CRT_UP and which is connected in parallelwith the eighth PMOS transistor.

Now, the operation of the level sensor 200 will be briefly described.

First, in the self refresh mode, when the self refresh interval signalSREF notifying that the current operation is in the self refresh mode isactivated, the level controller 240 is active to output any activatedone of the first and the second level control signals VBB_UP and VBB_DNas the up- or down-control signal CTR_UP or CTR_DN. Here, if the firstlevel adjustment signal VPP_UP is assumed to be set, the up-controlsignal CRT_UP is activated.

Then, the second NMOS transistor NM3 in the feedback circuit 220 isactive by the up-control signal CRT_UP, and therefore, although thelevel of the internal power VBB is the same, the level of the feedbackvoltage taken at the output end drops. Thus, the point of time at whichthe sensing signal BBE is activated is higher than that in the normalmode, so that the level of the internal power VBB rises.

Meanwhile, when the down-control signal CTR_DN is activated, the firstNMOS transistor NM2 in the feedback circuit 220 is active and the levelof the feedback voltage at the output end rises. Thus, the level of theinternal power VBB becomes lower than that in the normal mode.

As mentioned above, the internal power generator 100 including the levelsensor 200 adjusts the level of the internal power VBB based on thelevel adjustment signals VBB_UP and VBB_DN during the refresh mode, andthen outputs an adjusted internal power. That is, leakage current can beminimized by adjusting the level of the internal power VBB.

More concretely, in the leakage current generated in the memory cell, anoff-current or junction-current has relatively large importancedepending on the properties of product. In case an off-current is great,a retention time of cell data can be extended by lowering the level ofthe internal power VBB through the down-control signal CTR_DN during therefresh. Similarly, in case a junction-current is great, a retentiontime of cell data can be prolonged by elevating the level of theinternal power VBB through the up-control signal CTR_UP.

For reference, the level adjustment signals VBB_UP and VBB_DN are set inthe test step of wafer level so that the leakage current can beminimized. After this test procedure, the level-adjustment signalsVBB_UP and VBB_DN are set and applied through fuse option or metaloption.

As described above, the internal power generator adjusts the level ofthe internal power VBB during the self refresh mode, thereby decreasingleakage current of cell data. Accordingly, the retention time isprolonged.

Meanwhile, if the retention time of data is increased, the number oftimes of refresh can be decreased during the self refresh mode, therebyreducing IDD6 power consumption.

The following is a description for the refresh unit 500 capable ofadjusting the period of the self refresh on the basis of the adjustmentof the internal power VBB. For reference, the period adjustor 722 shownin FIG. 7 may be included in the refresh unit.

FIG. 10 is an internal circuit diagram of the refresh unit 500 depictedin FIG. 7.

Referring to FIG. 10, the refresh unit 500 includes a refresh exit/entrycontroller 600 for generating the self refresh interval signal SREFnotifying that the current operation is in the self refresh mode and theinternal auto refresh signal AREFP in response to the clock enablesignal CKE and the auto refresh command AREF_CMD, and a control signalgenerator 700 and 800 for periodically generating an internal refreshsignal REFP and an internal address RCNTI[0:N] during activation of theself refresh interval signal SREF, wherein an activation period of theinternal refresh signal REFP is varied upon variation of the level ofthe internal power VBB applied to a memory cell.

To be more specific, the refresh exit/entry controller 600 is providedwith a mode input/output controller 620 for taking the clock enablesignal CKE and the auto refresh signal AREF_CMD and generating theinternal auto refresh signal AREFP, a self refresh entry signal SREF_ENand a self refresh escape signal SREF_EXP, and a refresh interval signalgenerator 640 for generating the self refresh interval signal SREFnotifying a self refresh interval based on the internal auto refreshsignal AREFP, the self refresh entry signal SREF_EN and the self refreshescape signal SREF_EXP.

The control signal generator 700 and 800 is provided with a refreshperiod signal generator 720 for periodically outputting the period pulsesignal PL_FLG during activation of the self refresh interval signal SREFof which period is adjusted depending on the level of the internalpower, an internal refresh signal generator 740 for activating theinternal refresh signal REFP in response to the internal auto refreshsignal AREFP and the period pulse signal PL_FLG, and an internal addresscounter 800 for increasing a row address by one bit unit in response tothe internal refresh signal REFP to output the internal addressRCNTI[0:N].

As mentioned early, in case there is a variation of the level of theinternal power VBB determining the retention time of cell data, therefresh unit 500 of the invention reflects the above and adjusts theperiod of the internal refresh signal REFP. Thus, the period can bedelayed by the retention time of cell data, thereby reducing IDD6 powerconsumption.

FIG. 11 is an internal circuit diagram of the refresh period signalgenerator 720 of FIG. 10.

Referring to FIG. 11, the refresh period signal generator 720 isconstituted by the period adjustor 722 for generating a periodadjustment signal in response to the first and the second leveladjustment signals VBB_UP and VBB_DN, an oscillator 724 for adjustingthe period of the period signal OSC_OUT during activation of the selfrefresh interval signal SREF in response to the period adjustmentsignal, and a pulse generator 726 for generating the period-pulse signalPL_FLG of pulse type from the period signal OSC_OUT.

More specifically, the period adjustor 722 has a NOR gate NR1 for takingthe first and the second level adjustment signals VBB_UP and VBB_DN andoutputting the period adjustment signal.

The oscillator 724 is provided with a driving voltage supplier 724 a foradjusting a level of a driving voltage in response to the periodadjustment signal to supply an adjusted driving voltage and an inverterchain 724 b for producing the period signal OSC_OUT with a periodcorresponding to the level of the driving voltage during activation ofthe self refresh interval signal SREF.

The driving voltage supplier 724 a is composed of a plurality ofresistors coupled in series between the external voltage VDD and theground voltage VSS and an NMOS transistor NM4 whose gate receives theperiod adjustment signal and which is coupled in parallel with one ofthe plurality of resistors, wherein divided voltages are provided as afirst and a second driving voltages.

The pulse generator 726 is composed of a delay circuit 726 a fordelaying the period signal OSC_OUT, an inverter I3 for inverting anoutput signal of the delay circuit 726 a, an NAND gate ND3 taking anoutput signal of the inverter I3 and the period signal OSC_OUT, and aninverter I4 for inverting an output signal of the NAND gate ND3 toprovide the period pulse signal PL_FLG.

Now, the operation of the refresh period signal generator 720 will bebriefly explained.

First, when the first and the second level adjustment signals VBB_UP andVBB_DN are all inactivated, the period adjustment signal is inactivatedto a logic high level. Thus, the resistor, which is coupled in parallelwith the NMOS transistor NM4 that is under the control of the periodadjustment signal in the driving voltage supplier 724 a, does not affectthe total resistance value.

On the other hand, when one of the first and the second level adjustmentsignals VBB_UP and VBB_DN is activated, the period adjustor 722activates the period adjustment signal to a logic low level. Then, theNMOS transistor NM4 in the driving voltage supplier 724 a is turned off,which increases the total resistance value by the resistor coupled inparallel with it. Thus, the amount of current supplied from the drivingvoltage supplier 724 a is decreased and the period of the period signalOSC_OUT generated by the inverter chain 724 b is extended. Insuccession, the period of the period pulse signal PL_FLG created by thepulse generator 726 is also prolonged.

As such, the increase in period of the period pulse signal PL_FLG meansan increase in interval of the internal refresh signal REFP generated inresponse to it, and a small number of times of refresh is performed.

In other words, the level adjustment signals VBB_UP and VBB_DN aresignals that are set to adjust the level of the internal power VBB sothat the retention time of cell data is prolonged. Thus, the refreshunit 500 of the invention including the refresh period signal generator720 senses the state that the internal power VBB is adjusted by thelevel adjustment signals VBB_UP and VBB_DOWN and the retention time ofcell data is prolonged, and then extends the refresh period. The numberof times of refresh executed during the self refresh mode is reduced,which decreases IDD6 power consumption.

Therefore, the semiconductor memory device of the invention set forthabove adjusts the level of the internal power VBB so that the retentiontime of cell data is prolonged during the self refresh mode where onlythe refresh operation is conducted at regular intervals without anycommands from outside. Further, in case the level of the internal powerVBB is adjusted, the refresh interval is extended, thereby decreasingIDD6 power consumption.

As a result, the present invention adjusts the level of the internalpower VBB (bulk voltage applied to cell) so that the retention time ofcell data is prolonged during the self refresh mode and thus extends therefresh interval, thereby decreasing power being consumed during theself refresh mode.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A semiconductor memory device, comprising: a refresh exit/entry controller for taking a clock enable signal and an auto refresh command and generating a self refresh interval signal notifying that the current operation is in a self refresh mode; and a control signal generator for periodically generating an internal refresh signal and an internal address for a refresh driving during activation of the self refresh interval signal, wherein an activation period of the internal refresh signal varies upon a variation of a level of a bulk voltage applied to a memory cell.
 2. The semiconductor memory device as recited in claim 1, wherein the level variation of the bulk voltage is sensed depending on whether level adjustment signals being applied for level control of the bulk voltage are activated.
 3. The semiconductor memory device as recited in claim 2, wherein the control signal generator includes: a refresh period signal generator for periodically outputting a period-pulse signal during activation of the self refresh interval signal, the period of the period-pulse signal being adjusted in response to a level of the bulk voltage; an internal refresh signal generator for periodically outputting a period-pulse signal during activation of the self refresh interval signal, the internal refresh signal being activated in response to the period-pulse signal; and an internal address counter for increasing a row address by one bit unit in response to the internal refresh signal, to output the internal address.
 4. The semiconductor memory device as recited in claim 3, wherein the refresh period signal generator includes: a period adjustor for producing a period adjustment signal in response to the level adjustment signals; an oscillator for generating the period signal during activation of the self refresh interval signal, the period of the period signal being extended upon activation of the period adjustment signal; and a pulse generator for generating the period-pulse signal of pulse type whenever the period signal is activated.
 5. The semiconductor memory device as recited in claim 4, wherein the period adjustor includes a NOR gate for taking a first and a second level adjustment signals to output the period adjustment signal.
 6. The semiconductor memory device as recited in claim 5, wherein the oscillator includes: a driving voltage supplier for adjusting a level of a driving voltage in response to the period adjustment signal to supply an adjusted driving voltage; and an inverter chain for generating the period signal having a period corresponding to the level of the driving voltage during activation of the self refresh interval signal.
 7. The semiconductor memory device as recited in claim 6, wherein the driving voltage supplier includes: a plurality of resistors coupled in series between an external voltage and a ground voltage; an NMOS transistor whose gate takes the period adjustment signal and which is connected in parallel with one of the plurality of resistors, wherein voltages divided by the plurality of resistors are outputted as a first and a second driving voltages.
 8. A driving method of a semiconductor memory device, comprising: operating the semiconductor memory device in a self refresh mode by activating a self refresh interval signal in response to a clock enable signal and an auto refresh command; and generating an internal refresh signal and an internal address for controlling a refresh driving at predetermined intervals during an activation of the self refresh interval signal, wherein the self refresh interval signal notifies that a current operation is performed in the self refresh mode and the predetermined intervals are adjusted according to a level of a bulk voltage applied to a memory cell.
 9. The driving method as recited in claim 8, wherein the level variation of the bulk voltage is sensed depending on whether level adjustment signals being applied for level adjustment of the bulk voltage are activated. 